1. Field of the Invention:
The present invention relates to a variable speed gearing in a rotary electric tool such as a motor-driven screw-driver or drill.
2. Prior Art:
In conventional motor-driven screw-drivers and drills, the rotation of a terminal output shaft is generally varied between high and low speeds by changing the combination in the number of teeth of spur gears. An example is as disclosed in Japanese Patent Publication No. 15764/1984. In such conventional devices, however, the change of speed is performed manually using a speed changing handle, so that there is the inconvenience that the change over operation must be done during the rotation of the shaft.
More particularly, when the load exerted on the tool in use becomes large and the rotation stops, the speed is changed to low by operating the speed changing handle to increase the tightening torque. The degree of rotation of the tool is checked visually or is sensed by the hands of the user who holds the tool, and the speed is changed manually according to the thus-confirmed condition.